Direct conversion receivers, otherwise known as homodyne, synchrodyne, or zero IF receivers, have recently gained popularity for the reception of FM and other angle modulated signals due to their simplicity and potential for monolithic construction in integrated circuitry. They employ a radio receiver design that demodulates the incoming radio signal using mixer detection driven by a local oscillator whose frequency is equal to the carrier frequency of the signal being received.
The direct-conversion receiver feeds the radio signal into a frequency mixer, just as in a superheterodyne receiver, where it is mixed with a local oscillator signal. However, unlike the superheterodyne, the frequency of the local oscillator is set to the received signal's carrier frequency. The result is a demodulated output where the conversion to baseband is done in a single frequency conversion, avoiding the complexity of the superheterodyne's two (or more) frequency conversions, IF stage(s), and image rejection issues.
However, there are potential problems with the direct conversion receiver design. Signal leakage paths can occur where local oscillator energy can leak through the mixer stage and feed back to the antenna input where it reenters the mixer stage. The overall effect is that the local oscillator energy self-mixes and creates a DC offset signal. The offset can be large enough to overload the baseband amplifiers and swamp the wanted signal reception. To address these limitations, direct conversion receivers have typically include high pass filters, DC blocking capacitors, DC servo controllers, or other means to offset or remove the DC component. These DC blocking or offsetting components will produce a low frequency response cutoff point, reducing the magnitude of any information in the baseband below that low frequency response point. This phenomena is sometimes referred to as “the hole at DC” or “the hole in the middle”. For certain narrow band frequency modulation schemes, this low frequency cutoff will result in a significant loss of baseband information, resulting in increased distortion in the demodulated signal.
FIG. 1 is a block diagram of a conventional direct conversion receiver 100. The FM signal having a carrier frequency of fc enters the direct conversion receiver at antenna 102, is filtered by a bandpass filter 104, and is amplified by an amplifier 106. Following amplifier 106, the signal is split by a power divider 108 into two identical (except for amplitude) components. One component is sent to a mixer 114 and the other component sent to a mixer 116. A local oscillator 110, set to a frequency of fc, is split into two signals of different phase by phase splitter 112. The zero phase angle component leaving phase splitter 112 is directed to mixer 116, and the −90 degree phase angle component leaving phase splitter 112 is directed to mixer 114. The baseband components leaving mixers 116 and 114 are directed to high pass filters 120 and 118, respectively.
Hi pass filters 118 and 120 may be simple blocking capacitors, multi-component filter structures, DC servo feedback devices, etc., which will be referred to herein as “DC blockers.” The purpose of the DC blockers is to greatly attenuate (“remove”) DC created by leakage and feedback of local oscillator frequency fc that re-enters the circuitry via antenna 102 or mixers 114, 116. However, any DC blocker utilized in components 118 and 120 will have a cutoff frequency, above which it will substantially pass AC signals, and below which it begins to substantially attenuate them. If there are any low frequency components within the baseband that are below the cutoff frequency, they will be attenuated by the high pass filters 118 and 120, resulting in a loss of information in the demodulated baseband. Following high pass filters 118, 120, the signals are processed by low pass filters 122, 124, amplifiers 126, 128, and low pass filters 130, 132. The quadrature outputs are recombined in a device 134.
FIG. 2 is a graph 200 of SINAD as a function of Signal to Noise Ratio (SNR) for the typical prior art direct conversion receiver of FIG. 1. SINAD is the abbreviation for “signal noise and distortion” ratio, defined as [Psignal+Pnoise+Pdistortion]/[Pnoise+Pdistortion], where Psignal, Pnoise, and Pdistortion refer to signal power, noise power, and the power of the distortion components, respectively. The signal to noise ratio is commonly defined as SNR=Psignal/Pnoise. Curve 202 is obtained by sweeping the signal strength of a 1 KHz modulation tone FM signal (fed to antenna 102 of the circuit in FIG. 1) to create signal to noise ratios from 2 dB to 32 dB, while determining the SINAD values for the corresponding SNR. As can be seen, a limiting maximum value of the SINAD is about 24 dB, and has reached this value after SNR values of about 12-13 dB, for the conventional direct conversion receiver of FIG. 1.
FIG. 3 is a spectrum diagram 300 for the direct conversion receiver of FIG. 1 which illustrates the above-described performance problems due to the influence of distortion components. Curve 302 is the spectrum analysis plot of the same 1 KHz modulation tone FM signal used in FIG. 2. The spectrum analysis plot 302 shows two dominant peaks, one at 1 KHz (the fundamental tone) and one at 3 KHz, which is the third harmonic distortion level. This level of harmonic distortion is quite significant (being about −30 dB below the fundamental 1 KHz level) and explains the low level of SINAD of FIG. 2.
Prior attempts to address this problem include offsetting the local oscillator frequency by a fixed amount, typically close to that of the low frequency cutoff point of the DC blocking high pass filter. Since the cutoff frequencies may be on the order of a few Hertz to a few hundred Hz, control of the local oscillator fc to this level of precision is difficult, particularly if it must be maintained over a long period of time and ambient temperature variations.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.